For example, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
Normally, as a fuel gas supplied to the fuel cell, a hydrogen gas produced from a hydrocarbon based raw fuel by a reformer is used. In the reformer, the hydrocarbon based raw fuel such as a fossil fuel, e.g., methane or LNG is subjected to steam reforming or partial oxidation reforming, autothermal reforming or the like to produce a reformed gas (fuel gas).
For example, in the reformer for performing steam reforming, an evaporator is used for producing mixed vapor by mixing a raw fuel and water vapor. For example, Japanese Laid-Open Patent Publication No. 2000-16801 discloses an evaporator as shown in FIG. 11. The evaporator includes a double pipe 3 comprising an outer pipe 1 and an inner pipe 2. A plurality of heat conduction fins 1a are attached to the outer circumferential portion of the outer pipe 1, and spiral grooves 1b are formed in the inner circumferential portion of the outer pipe 1. A liquid supply pipe 1c is attached to an end of the outer pipe 1.
A sintered metal pipe 4 is interposed between the outer pipe 1 and the inner pipe 2 to form a vapor channel 5 between the sintered metal pipe 4 and the outer circumferential surface of the inner pipe 2. A heat medium channel 6 is formed in the inner pipe 2.
In the structure, since a hot gas 7a is supplied from the outside of the double pipe 3, the sintered metal pipe 4 is heated through the heat conduction fins 1a, and a heat medium 7b is supplied to the heat medium channel 6. A liquid to be vaporized (e.g., methanol, water, or mixed liquid of methanol and water) 8a is supplied to the liquid supply pipe 1c. When the liquid 8a is supplied to the spiral grooves 1b, the sintered metal pipe 4 is impregnated with the liquid 8a, and the liquid 8a is vaporized in the sintered metal pipe 4. Thus, the vapor 8b is supplied to the vapor channel 5.
In the case of using the evaporator for carrying out steam reforming in the solid oxide fuel cell, it is desirable that the hot exhaust gas discharged from the fuel cell is utilized as the hot gas 7a. It is because the temperature of the exhaust gas is considerably high (about 700° C.), and heat utilization ratio is improved by collecting the heat of the exhaust gas.
However, conditions such as the temperature and the flow rate of the exhaust gas change easily depending on the state of operation of the fuel cell. Thus, when the mixed fluid of the raw fuel (gas) and the water, or the water is supplied to the spiral grooves 1b as the liquid 8a to be vaporized, vaporization failure or boiling of water may occur, and vaporization and condensation may be repeated undesirably. Under the circumstances, fluctuation in the amount of produced water vapor occurs, and it is not possible to reliably obtain the mixed vapor (uniformly mixed raw fuel and water vapor). Further, pressure fluctuation (pulsation) in the mixed vapor occurs easily. Thus, at the time of producing a reformed gas by reforming the mixed vapor, the reforming reaction is not stable. The amount of electrical energy generated in the fuel cell fluctuates, and coking may occur due to the temporal shortage of water.